Marquis de Montsimmard foresaw the outbreak of the Mage-Templar War many years prior and when she exhibited magical talents, resolved not to let his daughter become a victim of it. Instead he sought to find protection for his daughter outside the Circle of Magi and the Templar Order.

That opportunity came at the outbreak of the War of the Lions, Lienne was sponsored into Grand Duke Gaspard de Chalons' service thanks to her prodigious talents with hexes and healing magic. Gaspard used her healing talents as a bargaining chip in order to extract Empress Celene's location from the potentially fatally wounded Comte Pierre.

Lienne continued to distinguish herself through the opening phase of the war. She escorted Gaspard everywhere, aiding him in battle and serving as his Arcane Advisor. Lienne advises Gaspard on the best tactics to combat the Sylvans who had set upon his Chevaliers as well as identifying the slaughter of the Dalish clan to be the work of a Demon on sight alone. Lienne remains with the entourage when Mihris leads Gaspard and his escort through the Eluvian in pursuit of Celene. When they finally confront Celene, the Empress identifies Lienne from her family resemblance. Celene made a point of mocking Lienne as naive for believing Gaspard would continue to protect an apostate once he had the entire Circle of Magi to serve him. Before Lienne could retaliate however she was rendered unconscious by Felassan.

Once Celene and Gaspard's entourage reach a truce as they travel through the Eluvian network, Lienne spends most of her time in the company Duke Remache of Lydes. After both groups reach the central chamber and slay the Varterral guarding it, Lienne assembles with Briala, Felassan, Mihris and Celene herself to observe the duel between Gaspard and Celene's champion, Ser Michel de Chevin to determine the fate of the Empire.

Although Gaspard forbade interference from his mages, Lienne discretely uses her hex magic to weaken Ser Michel while Mihris masks Lienne's magic and obscures the others from Gaspard and Michel's vision, allowing Gaspard to believe he won the duel fairly and Celene is making excuses. However, while revealing Mihris is possessed by Imshael, Briala uses the momentary distraction to stab Lienne in the stomach. Briala notes that Lienne could use her healing magic to save herself or could continue to aid Gaspard but lacks the power to do both. Rather than capitulate however Lienne swore she would see Briala dead. Briala comments that, in that case, she would be a fool to show Lienne mercy, and then slits Lienne's throat.